To understand the principles of taste coding, it is necessary to understand the functional organization of the taste organs. Although the labellum of the Drosophila melanogaster head has been described in detail, the tarsal segments of the legs, which collectively contain more taste sensilla than the labellum, have received much less attention. We performed a systematic anatomical, physiological, and molecular analysis of the tarsal sensilla of Drosophila. We construct an anatomical map of all five tarsal segments of each female leg. The taste sensilla of the female foreleg are systematically tested with a panel of 40 diverse compounds, yielding a response matrix of ∼500 sensillum–tastant combinations. Six types of sensilla are characterized. One type was tuned remarkably broadly: it responded to 19 of 27 bitter compounds tested, as well as sugars; another type responded to neither. The midleg is similar but distinct from the foreleg. The response specificities of the tarsal sensilla differ from those of the labellum, as do n-dimensional taste spaces constructed for each organ, enhancing the capacity of the fly to encode and respond to gustatory information. We examined the expression patterns of all 68 gustatory receptors (Grs). A total of 28 Gr–GAL4 drivers are expressed in the legs. We constructed a receptor-to-sensillum map of the legs and a receptor-to-neuron map. Fourteen Gr–GAL4 drivers are expressed uniquely in the bitter-sensing neuron of the sensillum that is tuned exceptionally broadly. Integration of the molecular and physiological maps provides insight into the underlying basis of taste coding.
Drosophila; Gr; gustatory receptor; legs; physiology; taste
Many insect vectors of disease detect their hosts through olfactory cues, and thus it is of great interest to understand better how odors are encoded. However, little is known about the molecular underpinnings that support the unique function of coeloconic sensilla, an ancient and conserved class of sensilla that detect amines and acids, including components of human odor that are cues for many insect vectors. Here, we generate antennal transcriptome databases both for wild type Drosophila and for a mutant that lacks coeloconic sensilla. We use these resources to identify genes whose expression is highly enriched in coeloconic sensilla, including many genes not previously implicated in olfaction. Among them, we identify an ammonium transporter gene that is essential for ammonia responses in a class of coeloconic olfactory receptor neurons (ORNs), but is not required for responses to other odorants. Surprisingly, the transporter is not expressed in ORNs, but rather in neighboring auxiliary cells. Thus, our data reveal an unexpected non-cell autonomous role for a component that is essential to the olfactory response to ammonia. The defective response observed in a Drosophila mutant of this gene is rescued by its Anopheles ortholog, and orthologs are found in virtually all insect species examined, suggesting that its role is conserved. Taken together, our results provide a quantitative analysis of gene expression in the primary olfactory organ of Drosophila, identify molecular components of an ancient class of olfactory sensilla, and reveal that auxiliary cells, and not simply ORNs, play an essential role in the coding of an odor that is a critical host cue for many insect vectors of human disease.
Olfaction underlies the attraction of insect pests and vectors of disease to their plant and human hosts. In the genetic model insect Drosophila, the neuronal basis of odor coding has been extensively analyzed in the antenna, its major olfactory organ, but the molecular basis of odor coding has not. Additionally, there has been little analysis of any olfactory cells other than neurons. We have undertaken a comprehensive and quantitative analysis of gene expression in the Drosophila antenna. This analysis revealed a surprisingly broad dynamic range of odor receptor and odor binding protein expression, and unexpected expression of taste receptor genes. Further analysis identified 250 genes that are expressed at reduced levels in a mutant lacking an evolutionarily ancient class of sensilla, antennal hairs housing neurons that respond to human odors. One of these genes, a transporter, is expressed in non-neuronal cells but is essential to the response of a neuron to ammonia, a key cue for insect vectors of disease. A mutation in this transporter can be rescued by its mosquito homolog. While many studies of sensory coding consider the neural circuit in isolation, our analysis reveals an essential role for an auxiliary cell.
Different species of fruit flies share habitats but are believed to mate with each other only rarely. In this issue, Fan et al. show that interspecies mating is inhibited by the taste receptor Gr32a (Gustatory receptor 32a) and a neural circuit in which it functions.
Odors elicit spatio-temporal patterns of activity in the brain. Spatial patterns arise from the specificity of the interaction between odorants and odorant receptors expressed in different olfactory receptor neurons (ORNs). But the origin of temporal patterns of activity and their role in odor coding remain unclear. We investigate how physiological aspects of ORN response and physical aspects of odor stimuli give rise to diverse responses in Drosophila ORNs. We show that odor stimuli have intrinsic dynamics that depend on odor type and strongly affect ORN response. Using linear-nonlinear modeling to remove the contribution of the stimulus dynamics from the ORN dynamics we study the physiological properties of the response to different odorants and concentrations. For several odorants and receptor types the ORN response dynamics normalized by the peak response are independent of stimulus intensity for a large portion of the neuron’s dynamic range. Adaptation to a background odor changes the gain and dynamic range of the response but does not affect normalized response dynamics. Stimulating ORNs with various odorants reveals significant odor-dependent delays in the ORN response functions. These differences however can be dominated by differences in stimulus dynamics. In one case the response of one ORN to two odorants is predicted solely from measurements of the odor signals. Within a large portion of their dynamic range ORNs can capture information about stimulus dynamics independently from intensity while introducing odor-dependent delays. How insects might use odor-specific stimulus dynamics and ORN dynamics in discrimination and navigation tasks remains an open question.
Diverse sensory organs, including mammalian taste buds and insect chemosensory sensilla, show a striking compartmentalization of receptor cells. However, the functional impact of this organization remains unclear. Here we show that compartmentalized Drosophila olfactory receptor neurons (ORNs) communicate with each other directly. The sustained response of one ORN is inhibited by the transient activation of a neighboring ORN. Mechanistically, such lateral inhibition does not depend on synapses and is likely mediated by ephaptic coupling. Moreover, lateral inhibition in the periphery can modulate olfactory behavior. Together, the results show that integration of olfactory information can occur via lateral interactions between ORNs. Inhibition of a sustained response by a transient response may provide a means of encoding salience. Finally, a CO2-sensitive ORN in the malaria mosquito Anopheles can also be inhibited by excitation of an adjacent ORN, suggesting a broad occurrence of lateral inhibition in insects and possible applications in insect control.
An intriguing question in the field of olfaction is how animals distinguish among structurally similar odorants. We systematically analyzed olfactory responses elicited by a panel of 25 pyrazines. We found that structurally similar pyrazines elicit a wide range of behavioral responses from Drosophila larvae. Each pyrazine was tested against all functional receptors of the larval Odor receptor (Or) repertoire, yielding 525 odorant–receptor combinations. Different pyrazines vary markedly in the responses they elicit from the Or repertoire, with most strong responses deriving from two receptors, Or33b and Or59a. Surprisingly, 2-ethylpyrazine and 2-methylpyrazine, which elicit strikingly similar physiological responses across the receptor repertoire, elicit dramatically different behavioral responses. A small fraction of odorant-receptor combinations elicit remarkably long responses. These responses, which we term “supersustained” responses, are receptor specific and odorant specific, and can last for minutes. Such supersustained responses may prevent olfactory neurons from reporting contemporaneous information about the local odor environment. Odors that elicit such responses could provide a novel means of controlling insect pests and vectors of human disease by impairing the location of human hosts, food sources, and mates.
Small animals like nematodes and insects analyze airborne chemical cues to infer the direction of favorable and noxious locations. In these animals, the study of navigational behavior evoked by airborne cues has been limited by the difficulty of precise stimulus control. We present a system that enables us to deliver gaseous stimuli in defined spatial and temporal patterns to freely moving small animals. We use this apparatus, in combination with machine vision algorithms, to assess and quantify navigational decision-making of Drosophila larvae in response to ethyl acetate (a volatile attractant) and carbon dioxide (a gaseous repellant).
Individual olfactory receptor neurons (ORNs) selectively express one or a small number of odor receptors from among a large receptor repertoire. The expression of an odor receptor dictates the odor response spectrum of the ORN. The process of receptor gene choice relies in part on a combinatorial code of transcription factors. In Drosophila, the POU domain transcription factor Acj6 is one element of the transcription factor code. In acj6 null mutants, many ORNs do not express an appropriate odor receptor gene and thus are not correctly specified. We find that acj6 is alternatively spliced to yield many structurally distinct transcripts in the olfactory organs. We generate flies that express single splice forms of acj6 in an acj6− background. We find that different splice forms are functionally distinct; they differ in their abilities to specify ORN identities. Some individual splice forms can fully rescue the specification of some ORNs. Individual splice forms can function both positively and negatively in receptor gene regulation. ORNs differ in their requirements for splice forms; some are not fully rescued by any single splice form tested, suggesting that some ORNs may require the combinatorial action of multiple splice forms. Late expression of some acj6 splice forms is sufficient to rescue some ORN classes, consistent with a direct role for Acj6 isoforms in receptor gene expression. The results indicate that alternative splicing may add another level of richness to the regulatory code that underlies the process of odor receptor gene choice.
olfaction; Drosophila; Acj6; POU-domain; odor receptor; splicing
A central question in insect chemoreception is whether signaling occurs via G-proteins. Two families of seven-transmembrane-domain chemoreceptors, the Or and Gr receptor families, have been identified in Drosophila (Clyne et al., 1999; Vosshall et al., 1999; Clyne et al., 2000). Or receptors mediate odor responses while two Gr receptors, Gr21a and Gr63a, mediate CO2 response (Hallem et al., 2004; Jones et al., 2007; Kwon et al., 2007). Using single-sensillum recordings, we systematically investigate the role of Gα proteins in vivo, initially with RNAi constructs, competitive peptides, and constitutively active Gα proteins. The results do not support a role for Gα proteins in odor sensitivity. In parallel experiments, manipulations of Gαq, but not other Gα proteins, affected CO2 response. Transient, conditional, and ectopic expression analyses consistently supported a role for Gαq in the response of CO2-sensing neurons, but not odor-sensing neurons. Genetic mosaic analysis confirmed that odor responses are normal in the absence of Gαq. Gγ30A is also required for normal CO2 response. The simplest interpretation of these results is that Gαq and Gγ30A play a role in the response of CO2-sensing neurons, but are not required for Or-mediated odor signaling.
Olfactory; Drosophila; Transduction; Receptor; GPCR; signal transduction
Little is known about how individual olfactory receptor neurons (ORNs) select, from among many odor receptor genes, which genes to express. Acj6 (Abnormal chemosensory jump 6) is a POU-domain transcription factor essential for the specification of ORN identity and Or (odor receptor) gene expression in the Drosophila maxillary palp, one of the two adult olfactory organs. However, the mechanism by which Acj6 functions in this process has not been investigated. Here we systematically examine the role of Acj6 in the maxillary palp and in a major subset of antennal ORNs. We define an Acj6 binding site by a reiterative in vitro selection process. The site is found upstream of Or genes regulated by Acj6, and Acj6 binds to the site in Or promoters. Mutational analysis shows that the site is essential for Or regulation in vivo. Surprisingly, a novel ORN class in acj6 adults is found to arise from ectopic expression of a larval Or gene, which is repressed in wild-type via an Acj6 binding site. Thus Acj6 acts directly in the process of receptor gene choice; it plays a dual role, positive and negative, in the logic of the process, and acts in partitioning the larval and adult receptor repertoires.
olfaction; Drosophila; Acj6; POU-domain; maxillary palp; antenna
To explore differences in cerebral oxygen reserves during sleep in old and young adults
Descriptive cross-sectional study
General Clinical Research Center
Nine old (65–84yrs) and 10 young (21–39yrs) adults
Subjects were monitored during the first nightly sleep cycle using standard polysomnography, including measures of arterial oxyhemoglobin saturation (SaO2). Changes in regional cerebral oxyhemoglobin saturation (rcSO2) were used to estimate cerebral oxygen reserves. General linear models were used to test group differences in the change in SaO2 and rcSO2 during sleep.
Compared to young subjects, the old had reduced SaO2, both before sleep (baseline) (F(1,18)=5.1, p=.04) and when asleep (F(1,18)=5.14, p=.04). During sleep, half of the old and none of the young had SaO2 values below 95%. In addition, the old had more periods of oxygen desaturation (drops in SaO2≥4%) (X2=24.3, p=.01) and lower SaO2 levels during desaturation (F(1,18)=11.11, p<.01). Although baseline values were similar, rcSO2 decreased during sleep by 2.1% in the old (F(1,8)=3.8, p=.05) but increased by 2.1% during sleep in the young (F(1,9)=4.6, p=.04). When the old awakened from sleep, the rcSO2, but not the SaO2, returned to baseline; both returned to baseline in the young.
This exploratory analysis generates the hypothesis that lower SaO2, combined with declines in regional blood flow, contributes to the decline in cerebral oxygen reserves during sleep in the old. Further study will assess the effects of factors (e.g. medical conditions, subclinical disorders, and sleep architecture) that might account for these differences.
Brain Hypoxia; Hypoxemia; Aging; Sleep; Oximetry
The aim of this descriptive exploratory study was to describe patterns of cerebral oxygen reserves during sleep and their association with cerebrovascular risk factors in elders.
Participants--115 elders, age 70+ years--were monitored overnight using standard polysomnography. Measures included arterial oxyhemoglobin (SaO2) and regional measures of percent cerebral oxyhemoglobin saturation (rcSO2) via cerebral oximetry. Subjects were classified based on the magnitude of change in rcSO2 from resting baseline to the end of the first non-rapid-eye-movement (NREM) period. One way ANOVA and Chi-square were used to test group differences in SaO2 and the prevalence of cerebrovascular risk factors.
20 subjects (Group 1) experienced an increase in rcSO2 during sleep along with sleeping rcSO2 levels ≥ 55%; 95 subjects experienced a decline in rcSO2; 72 subjects (Group 2) had sleeping rcSO2 levels ≥ 55%; and 23 subjects had sleeping rcSO2 levels < 55% (Group 3). Although all three groups had equivalent declines in SaO2 levels during sleep, Group 3 had more cardiovascular comorbidity than Groups 1 and 2.
While SaO2 levels decline in most people during sleep, compensatory vascular responses to these drops in SaO2 are important for preventing rcSO2 from falling during sleep. Those entering sleep with lower baseline rcSO2 levels and those with greater declines in cerebral oxygenation during sleep may have greater cardiovascular burden and be at greater risk for stroke and other forms of disabling cerebrovascular disease.
Aging; Sleep; Oxygenation; Cerebral; Arterial; Blood Flow; Instrumentation
Many studies attest to the challenges of recruiting and retaining older adults in longitudinal studies. This article presents the methods used by the Physiological Research to Improve Sleep and Memory Project to recruit and retain 115 adults (70+ years) in a 2-year study that involved yearly administrations of two neurocognitive test batteries and two nights of polysomnography. The paper describes strategies which are built on knowledge obtained from participant informants and the use of tailored, individualize protocols. Together, these strategies enabled participants to become vested in the research process and to fully participate in all aspects of the study.
Research Subjects; Cognition; Sleep; Frail Elders; Prospective Studies
We examine the molecular and cellular basis of taste perception in the Drosophila larva, through a comprehensive analysis of the expression patterns of all 68 Gustatory receptors (Grs). Gr-GAL4 lines representing each Gr are examined, and 39 show expression in taste organs of the larval head, including the terminal organ (TO), the dorsal organ (DO), and the pharyngeal organs. A receptor-to-neuron map is constructed. The map defines 10 neurons of the TO and DO, and it identifies 28 receptors that map to them. Each of these neurons expresses a unique subset of Gr-GAL4 drivers, except for two neurons that express the same complement. All of these neurons express at least two drivers, and one neuron expresses 17. Many of the receptors map to only one of these cells, but some map to as many as six. Conspicuously absent from the roster of Gr-GAL4 drivers expressed in larvae are those of the sugar receptor subfamily. Coexpression analysis suggests that most larval Grs act in bitter response, and that there are distinct bitter-sensing neurons. A comprehensive analysis of central projections confirms that sensory information collected from different regions, e.g. the tip of the head vs the pharynx, is processed in different regions of the suboesophageal ganglion (SOG), the primary taste center of the central nervous system. Taken together, the results provide an extensive view of the molecular and cellular organization of the larval taste system.
The extent of diversity among bitter-sensing neurons is a fundamental issue in the field of taste. Data are limited and conflicting as to whether bitter neurons are broadly tuned and uniform, resulting in indiscriminate avoidance of bitter stimuli, or diverse, allowing a more discerning evaluation of food sources. We provide a systematic analysis of how bitter taste is encoded by the major taste organ of the Drosophila head, the labellum. Each of 16 bitter compounds is tested physiologically against all 31 bitter neurons, revealing responses that are diverse in magnitude and dynamics. Four functional classes of bitter neurons are defined. Four corresponding classes are defined through expression analysis of all 68 Gr taste receptors. A receptor-to-neuron-to-tastant map is constructed. Misexpression of one receptor confers bitter responses as predicted by the map. These results reveal a degree of complexity that greatly expands the capacity of the system to encode bitter taste.
This descriptive cross-sectional study investigated the relationships between cerebral oxygen reserve and cognitive function in community-dwelling older adults.
Participants (72 women and 40 men) underwent standard polysomnography, including regional measures of percent oxyhemoglobin saturation (rcSO2) determined by cerebral oximetry. Two variables were used to calculate cerebral oxygen reserve: (a) awake rcSO2 (mean presleep rcSO2) and (b) the change in rcSO2 from before sleep to the end of the first non-rapid-eye movement cycle. General linear models, adjusted for the effects of education and occupation, tested differences in performance on standard tests of memory, attention, and speed of mental processing.
Awake rcSO2 values were normal (60%–79.9%) in 64 participants, marginal (50%–59.9%) in 41, and low (43%–49.9%) in 7. Participants with normal awake levels had higher cognitive function than those with low levels (p < .05). Changes in rcSO2 were greatest in participants with marginal awake rcSO2 values; among whom, those who increased rcSO2 during sleep (n = 17) had better memory function than the 24 who did not (p < .05).
Low awake rcSO2 values mark individuals with low cerebral oxygen reserves and generally lower cognitive function; marginal awake rcSO2 values that fall during sleep may indicate loss of cerebral oxygen reserve and an increased risk for cognitive decline. Further studies may clarify the significance of and mechanisms underlying individual differences in awake rcSO2 and the changes that occur in rcSO2 while asleep.
Cerebral oxygenation; Sleep; Cognition
Variability in disease-related outcomes may relate to how patients experience self-management support in clinical settings.
To identify factors associated with experiences of self-management support during primary care encounters.
A cross-sectional survey was conducted of 208 patients seen in a multidisciplinary diabetes program in an academic medicine clinic. Multiple regression analysis was used to test associations between patient-rated experiences of self-management support (Patient Assessment of Chronic Illness Care [PACIC]) and race, gender, insurance status, literacy, duration of diabetes, and intensity of care management.
The PACIC ratings decreased with age (r = −0.235, p = .001), were higher for women than for men (3.95 vs. 3.65, t = 2.612, p = .010), and were greater for those with more education (F = 3.927, p = .009) and greater literacy skills (t = 3.839, p < .001). The ratings did not vary between racial (t = −1.108, p = .269) or insurance (F = 1.045, p = .374) groups and were unaffected by duration of diabetes (r = 0.052, p = .466) and the intensity of care management (F = 1.028, p = .360). In multivariate models, literacy was the only variable contributing significantly to variation in self-management support ratings.
Even when considering the objective intensity of health services delivered, literacy was the sole variable contributing to differences in patient ratings of self-management support. Although conclusions are limited by the cross-sectional nature of this study, the results emphasize the need to consider literacy when developing and communicating treatment plans requiring self-management skills.
diabetes mellitus; self-care; literacy
Many species of mosquitoes, including the major malaria vector Anopheles gambiae, utilize carbon dioxide (CO2) and 1-octen-3-ol as olfactory cues in host-seeking behaviors that underlie their vectorial capacity. However, the molecular and cellular basis of such olfactory responses remains largely unknown.
Here, we use molecular and physiological approaches coupled with systematic functional analyses to define the complete olfactory sensory map of the An. gambiae maxillary palp, an olfactory appendage that mediates the detection of these compounds. In doing so, we identify three olfactory receptor neurons (ORNs) that are organized in stereotyped triads within the maxillary-palp capitate-peg-sensillum population. One ORN is CO2-responsive and characterized by the coexpression of three receptors that confer CO2 responses, whereas the other ORNs express characteristic odorant receptors (AgORs) that are responsible for their in vivo olfactory responses.
Our results describe a complete and highly concordant map of both the molecular and cellular olfactory components on the maxillary palp of the adult female An. gambiae mosquito. These results also facilitate the understanding of how An. gambiae mosquitoes sense olfactory cues that might be exploited to compromise their ability to transmit malaria.
Remarkable advances in our understanding of olfactory perception have been made in recent years, including the discovery of new mechanisms of olfactory signaling and new principles of olfactory processing. Here we discuss the insight that has been gained into the receptors, cells, and circuits that underlie the sense of smell.
The mosquito Anopheles gambiae is the major vector of malaria in sub-Saharan Africa. It locates its human hosts primarily through olfaction, but little is known about the molecular basis of this process. Here we functionally characterize the Anopheles gambiae Odourant Receptor (AgOr) repertoire. We identify receptors that respond strongly to components of human odour and that may act in the process of human recognition. Some of these receptors are narrowly tuned, and some salient odourants elicit strong responses from only one or a few receptors, suggesting a central role for specific transmission channels in human host-seeking behavior. This analysis of the Anopheles gambiae receptors permits a comparison with the corresponding Drosophila melanogaster odourant receptor repertoire. We find that odourants are differentially encoded by the two species in ways consistent with their ecological needs. Our analysis of the Anopheles gambiae repertoire identifies receptors that may be useful targets for controlling the transmission of malaria.
During the time when preterm infants' oral feeding skills are developing they often experience physiological instability and need assistance from caregivers to maintain adequate oxygenation. Assisting infants to maintain optimal oxygenation during oral feeding requires an understanding of how they express and aim to self-regulate their oxygen status.
The purpose of this study was to identify potential behavioural indicators of declining oxygenation during preterm infant early bottle-feeding.
The design was explorative. Data were collected from a secondary analysis of 20 videotapes of preterm infant bottle feedings which included concurrent oxygen saturation data. In this analysis infant behaviours and quality of breathing were coded and compared across three periods: high oxygen saturation, immediately preceding an oxygen desaturation event, and during an oxygen desaturation event.
Infants gave limited behavioural indicators of declining oxygenation. Immediately prior to a desaturation event, they had an increase in eye flutter and were typically sucking and apnoeic. During a desaturation event, they typically relaxed their arms/hands and stopped sucking.
Reliance on preterm infant behavioural cues will be insufficient for detection of oxygen desaturation during oral feeding. Attention to changes in breath sounds and to the pattern of sucking are potentially important intervention strategies for the prevention of and appropriate response to oxygen decline during feeding. Sucking pauses may be a time when preterm infants aim to regulate their breathing pattern and thereby increase oxygenation. Interventions that focus on detection and minimization of apnoea during feeding, and which aim to protect infant sucking pauses, may reduce the number and severity of desaturation events preterm infants experience during bottle feeding.
preterm; oxygenation; desaturation; oral feeding; bottle feeding; sucking; energy; breathing; nursing
We investigate the logic by which sensory input is translated into behavioral output. First we provide a functional analysis of the entire odor receptor repertoire of an olfactory system. We construct tuning curves for the 21 functional odor receptors of the Drosophila larva, and show that they sharpen at lower odor doses. We construct a 21-dimensional odor space from the responses of the receptors and find that the distance between two odors correlates with the extent to which one odor masks the other. Mutational analysis shows that different receptors mediate the responses to different concentrations of an odorant. The summed response of the entire receptor repertoire correlates with the strength of the behavioral response. The activity of a small number of receptors is a surprisingly powerful predictor of behavior. Odors that inhibit more receptors are more likely to be repellents. Odor space is largely conserved between two dissimilar olfactory systems.
Olfaction; Drosophila; odor receptor; larva; behavior
Olfaction depends on the differential activation of olfactory receptor neurons (ORNs) and on the proper transmission of their activities to the brain. ORNs select individual receptors to express, and they send axons to particular targets in the brain. Little is known about the molecular mechanisms underlying either process. We have identified a new Drosophila POU gene, pdm3, that is expressed in ORNs. Genetic analysis shows that pdm3 is required for odor response in one class of ORNs. We find that pdm3 acts in odor receptor expression in this class, and that the odor response can be rescued by the receptor. Another POU gene, acj6, is required for receptor expression in the same class, and we find a genetic interaction between the two POU genes. The results support a role for a POU gene code in receptor gene choice. pdm3 is also expressed in other ORN classes in which it is not required for receptor expression. For two of these classes pdm3 is required for normal axon targeting. Thus this mutational analysis, the first for a POU class VI gene, demonstrates a role for pdm3 in both of the processes that define the functional organization of ORNs in the olfactory system.
POU gene; odor receptor; axon targeting; Drosophila; maxillary palp; antenna
By analogy to mammals, odorant receptors (ORs) in insects, such as Drosophila melanogaster, have long been thought to belong to the G-protein coupled receptor superfamily. However, recent work has cast doubt on this assumption and has tentatively suggested an inverted topology compared to the canonical Nout-Cin 7TM GPCR topology, at least for some Drosophila ORs. Here, we report a detailed topology mapping of the Drosophila OR83b receptor using engineered glycosylation sites as topology markers. Our results are inconsistent with a classical GPCR topology and show that OR83b has an intracellular N-terminus, an extracellular C-terminus, and 7 transmembrane helices.
odorant receptor; Drosophila melanogaster; membrane protein; topology
We have analyzed the molecular basis of sugar reception in Drosophila. We define the response spectrum, concentration dependence, and temporal dynamics of sugar-sensing neurons. Using in situ hybridization and reporter gene expression we identify members of the Gr5a-related taste receptor subfamily that are co-expressed in sugar neurons. Neurons expressing different Gr5a-related genes send overlapping but distinct projections to the brain and thoracic ganglia. Genetic analysis of receptor genes shows that Gr5a is required for response to one subset of sugars and Gr64a for response to a complementary subset. A Gr5a;Gr64a double mutant shows no physiological or behavioral responses to any tested sugar. The simplest interpretation of our results is that Gr5a and Gr64a are each capable of functioning independently of each other within individual sugar neurons and that they are the primary receptors used in the labellum to detect sugars.